Several publications and patent documents are cited throughout the specification in order to describe the state of the art to which this invention pertains. Each of these citations is incorporated by reference herein as though set forth in full.
Resistance to programmed cell death (apoptosis) is a characteristic of many cancer cells. Accordingly, efforts to develop new treatment strategies and therapeutic targets include attempts to identify and characterize proteins that regulate other survival or stress-response pathways. Heat shock proteins (HSPs) are encoded by evolutionarily conserved gene families and are required for cell survival following various forms of stress. HSPs are generally classified according to their approximate molecular size, and are structurally and functionally diverse; some are constitutively expressed, while others are stress-induced (Mayer and Bukau, 2005; Brodsky and Chiosis, 2006; Garrido et al., 2006; Schmitt et al., 2006; Powers and Workman, 2007). The stress-inducible protein HSP70 (also called HSP72, HSP70-1 or HSPA1A) is an approximately 70 kDa ATP-dependent molecular chaperone that is present at low or undetectable levels in most unstressed normal cells and tissues. Its abundance rapidly increases in response to a variety of metabolic or exogenous insults that, among other effects, can cause changes in protein conformation or stability. HSP70-inducing stresses include elevated temperatures, nutrient deprivation, heavy metals, oxidative stress and viral infections. The stress-inducible HSP70 is thought to help cells cope with these potentially deleterious conditions, in part by aiding with folding of nascent polypeptides or refolding of damaged proteins, preventing/reversing protein aggregation or self-association, promoting protein transport to intracellular locations for degradation, and aiding in the formation of protein complexes. HSP70 also is an important regulator of apoptotic signaling pathways, acting in part through direct interactions with substrate proteins that affect multiple steps in the process, including control of mitochondrial membrane integrity and caspase-activation (Mayer and Bukau, 2005; Brodsky and Chiosis, 2006; Garrido et al., 2006; Schmitt et al., 2006; Powers and Workman, 2007).
In contrast to its low abundance in unstressed normal cells, the inducible HSP70 protein is present at constitutively elevated levels in many human tumors of various origin. Such enhanced HSP70 expression correlates with resistance of the tumor cells to caspase-dependent and -independent cell death and is associated with poor patient prognosis (Brodsky and Chiosis, 2006; Garrido et al., 2006; Guzhova and Margulis, 2006; Schmitt et al., 2006). It is likely that the unfavorable conditions associated with the tumor microenvironment, such as hypoxia, nutrient deprivation, oxidative stress, oncogene activation, and exposure to chemotherapeutics lead to alterations in protein structure or processing, as well as upregulation of HSP70. The actions of this protein would be expected to help tumor cells tolerate, or adapt to, these conditions, and current evidence suggests that elevated HSP70 expression promotes tumorigenesis. Conversely, reducing HSP70 levels in some cultured tumor cells has been reported to induce cell death, and/or to sensitize them to cytotoxic agents, while having no obvious deleterious effects on non-tumor cells (Nylandsted et al., 2000, 2002; Rohde et al., 2005; Schmitt et al., 2006; Aghdassi et al., 2007; Powers et al., 2008).
In addition to its cytoprotective actions in promoting tumorigenesis, an altered expression or function of HSP70 also has been implicated in certain other human disorders that are associated with defects in protein conformation or folding. This includes disorders caused by the presence of mutant proteins, as well as some neurodegenerative diseases such as Alzheimer's and Parkinson's disease, and viral pathogenesis (McClellan et al., 2005; Muchowski and Wacker, 2005; Brodsky and Chiosis, 2006; Guzhova and Margulis, 2006; Morimoto, 2008). The identification of small molecules that specifically interact with, and modulate the activities of, HSP70 therefore has important implications for a number of human diseases. To date, however, only a limited number of compounds that specifically target HSP70 have been identified in chemical screens, and few are currently available to assess the physiologic impact of modulating HSP70 actions (Brodsky and Chiosis, 2006; Powers and Workman, 2007; Wisén and Gestwicki, 2008).